It is known that various blood parameters may be calculated by measuring the transmittance of light at different wavelengths through tissue having blood flowing therein. Examples of such blood parameters include carbon monoxide, carbon dioxide, glucose, and oxygen concentrations. Accurate information on these blood parameters may be important for a variety of reasons. For example, in the operating room, up-to-date information regarding oxygen saturation can be used to signal changing physiological factors, the malfunction of anaesthesia equipment, or physician error. Similarly, in the intensive care unit, oxygen saturation information can be used to confirm the provision of proper patient ventilation and allow the patient to be withdrawn from a ventilator at an optimal rate.
The non-invasive technique of measuring light transmittance in order to formulate blood parameter information is desirable in many applications for reasons of operator convenience and patient comfort. One well known technique that determines oxygen saturation in blood is pulse transmittance oximetry. The technique generally involves measuring the transmittance of light through body tissue at two different wavelengths. Typically, the two wavelengths are in the red and infrared regions. The measurements are made at both systolic pressure and diastolic pressure. In one known formulation, an oxygen saturation ratio is given by: ##EQU1## where R.sub.OS is the oxygen saturation ratio, R.sub.L is the transmittance of light at the red wavelength at systolic pressure, R.sub.H is the transmittance of light at the red wavelength at diastolic pressure, IR.sub.L is the transmittance of light at the infrared wavelength at systolic pressure, and IR.sub.H is the transmittance of light at the infrared wavelength at diastolic pressure. Oxygen saturation may then be ascertained from the R.sub.OS value using empirically derived calibration curves. The precise description of the method and apparatus for measuring the transmittance of light is not part of the present invention and so is described here only generally. Reference to U.S. Pat. No. 4,819,646 to Cheung et al. is recommended for a detailed description of pulse transmittance oximetry.
The accuracy of R.sub.OS is dependent therefore on the accuracy of the measurements of the transmittance of light at both wavelengths and at both systolic and diastolic pressure. The transmittance of light measurements are detected typically by a photodiode. One significant difficulty with transmittance of light measurements is the introduction of noise. Noise may originate from several sources including, but not limited to: preamplifier noise, induced noise from inside the oximeter, induced noise from outside the oximeter, and ambient light noise.
The present invention provides a pulse transmittance oximeter that is insensitive to noise.